checking if an object IS in a list

[nicolas.pourcelot]

Hi,

I want to test if an object IS in a list (identity and not equality
test).
I can if course write something like this :

test = False
myobject = MyCustomClass(* args, **kw)
for element in mylist:
if element is myobject:
test = True
break

and I can even write a isinlist(elt, mylist) function.

But most of the time, when I need some basic feature in python, I
discover later it is in fact already implemented. ;-)

So, is there already something like that in python ?
I tried to write :
'element is in mylist'
but this appeared to be incorrect syntax...

All objects involved all have an '__eq__' method.
Thanks,

N. P.

PS: Btw, how is set element comparison implemented ? My first
impression was that 'a' and 'b' members are considered equal if and
only if hash(a) == hash(b), but I was obviously wrong :

>>class A(object):

.... def __eq__(self,y):
.... return False
.... def __hash__(self):
.... return 5
....

>>a=A();b=A()
a==b

False

>>hash(b)==hash (a)

True

>>b in set([a])

False

>>S=set([a])
S.differenc e([b])

set([<__main__.A object at 0xb7a91dac>])

So there is some equality check also, maybe only if '__eq__' is
implemented ?

[Peter Otten]

ni************* **@gmail.com wrote:

Hi,

I want to test if an object IS in a list (identity and not equality
test).
I can if course write something like this :

test = False
myobject = MyCustomClass(* args, **kw)
for element in mylist:
if element is myobject:
test = True
break

and I can even write a isinlist(elt, mylist) function.

But most of the time, when I need some basic feature in python, I
discover later it is in fact already implemented. ;-)

So, is there already something like that in python ?
I tried to write :
'element is in mylist'
but this appeared to be incorrect syntax...

There is no "is in" operator in Python, but you can write your test more
concisely as

any(myobject is element for element in mylist)

PS: Btw, how is set element comparison implemented ? My first
impression was that 'a' and 'b' members are considered equal if and
only if hash(a) == hash(b), but I was obviously wrong :

>>>class A(object):

... def __eq__(self,y):
... return False
... def __hash__(self):
... return 5
...

>>>a=A();b=A( )
a==b

False

>>>hash(b)==has h(a)

True

>>>b in set([a])

False

>>>S=set([a])
S.difference ([b])

set([<__main__.A object at 0xb7a91dac>])

So there is some equality check also, maybe only if '__eq__' is
implemented ?

In general equality is determined by __eq__() or __cmp__(). By default
object equality checks for identity.

Some containers (like the built-in set and dict) assume that a==b implies
hash(a) == hash(b).

Peter

[nicolas.pourcelot]

On 18 juil, 11:30, Peter Otten <__pete...@web. dewrote:

nicolas.pource. [e-mail] wrote:

Hi,

I want to test if an object IS in a list (identity and not equality
test).
I can if course write something like this :

test = False
myobject = MyCustomClass(* args, **kw)
for element in mylist:
* * if element is myobject:
* * * * test = True
* * * * break

and I can even write a isinlist(elt, mylist) function.

But most of the time, when I need some basic feature in python, I
discover later it is in fact already implemented. ;-)

So, is there already something like that in python ?
I tried to write :
'element is in mylist'
but this appeared to be incorrect syntax...

There is no "is in" operator in Python, but you can write your test more
concisely as

any(myobject is element for element in mylist)

Thanks a lot
However, any() is only available if python version is >= 2.5, but I
may define a any() function on initialisation, if python version < 2.5

I think something like

>>id(myobject ) in (id(element) for element in mylist)

would also work, also it's not so readable, and maybe not so fast
(?)...

An "is in" operator would be nice...

PS: Btw, how is set element comparison implemented ? My first
impression was that 'a' and 'b' members are considered equal if and
only if hash(a) == hash(b), but I was obviously wrong :

>>class A(object):

... def __eq__(self,y):
... return False
... def __hash__(self):
... return 5
...

>>a=A();b=A()
a==b

False

>>hash(b)==hash (a)

True

>>b in set([a])

False

>>S=set([a])
S.differenc e([b])

set([<__main__.A object at 0xb7a91dac>])

So there is some equality check also, maybe only if '__eq__' is
implemented ?

In general equality is determined by __eq__() or __cmp__(). By default
object equality checks for identity.

Some containers (like the built-in set and dict) assume that a==b implies
hash(a) == hash(b).

Peter

So, precisely, you mean that if hash(a) != hash(b), a and b are
considered distinct, and else [ie. if hash(a) == hash(b)], a and b are
the same if and only if a == b ?

[nicolas.pourcelot]

In fact, 'any(myobject is element for element in mylist)' is 2 times
slower than using a for loop, and 'id(myobject) in (id(element) for
element in mylist)' is 2.4 times slower.

[Peter Otten]

ni************* **@gmail.com wrote:

I think something like

>>>id(myobjec t) in (id(element) for element in mylist)

would also work, also it's not so readable, and maybe not so fast
(?)...

An "is in" operator would be nice...

And rarely used. Probably even less than the (also missing)

< in, | in, you-name-it

operators...

So, precisely, you mean that if hash(a) != hash(b), a and b are
considered distinct, and else [ie. if hash(a) == hash(b)], a and b are
the same if and only if a == b ?

Correct for set, dict. For lists etc. the hash doesn't matter:

>>class A(object):

.... def __hash__(self):
.... return nexthash()
.... def __eq__(self, other):
.... return True
....

>>from itertools import count
nexthash = count().next
A() in [A() for _ in range(3)]

True

>>d = dict.fromkeys([A() for a in range(3)])
d.keys()[0] in d

False

Peter

[Peter Otten]

ni************* **@gmail.com wrote:

In fact, 'any(myobject is element for element in mylist)' is 2 times
slower than using a for loop, and 'id(myobject) in (id(element) for
element in mylist)' is 2.4 times slower.

This is not a meaningful statement unless you at least qualify with the
number of item that are actually checked. For sufficently long sequences
both any() and the for loop take roughly the same amount of time over here.

$ python -m timeit -s"items=range(1 000); x = 1000" "any(x is item for item
in items)"
1000 loops, best of 3: 249 usec per loop
$ python -m timeit -s"items=range(1 000); x = 1000" "for item in items:" "
if x is item: break"
1000 loops, best of 3: 276 usec per loop

$ python -m timeit -s"items=range(1 000); x = 0" "any(x is item for item in
items)"
100000 loops, best of 3: 3 usec per loop
$ python -m timeit -s"items=range(1 000); x = 0" "for item in items:" " if x
is item: break"
1000000 loops, best of 3: 0.317 usec per loop
Peter

PS: Take these numbers with a grain of salt, they vary a lot between runs.

[nicolas.pourcelot]

On 18 juil, 12:26, Peter Otten <__pete...@web. dewrote:

nicolas.pource. [e-mail] wrote:

I think something like

>>id(myobject ) in (id(element) for element in mylist)

would also work, also it's not so readable, and maybe not so fast
(?)...

An "is in" operator would be nice...

And rarely used. Probably even less than the (also missing)

< in, | in, you-name-it

operators...

Maybe, but imho

>>myobject is in mylist

is highly readable, when

>>myobject < in mylist

is not.

[nicolas.pourcelot]

On 18 juil, 13:13, Peter Otten <__pete...@web. dewrote:

nicolas.pource. [e-mail] wrote:

In fact, 'any(myobject is element for element in mylist)' is 2 times
slower than using a for loop, and 'id(myobject) in (id(element) for
element in mylist)' is 2.4 times slower.

This is not a meaningful statement unless you at least qualify with the
number of item that are actually checked. For sufficently long sequences
both any() and the for loop take roughly the same amount of time over here.

Sorry. I used short lists (a list of 20 floats) and the element
checked was not in the list.
(That was the case I usually deals with in my code.)

[Peter Otten]

ni************* **@gmail.com wrote:

On 18 juil, 13:13, Peter Otten <__pete...@web. dewrote:

>nicolas.pource [e-mail] wrote:

In fact, 'any(myobject is element for element in mylist)' is 2 times
slower than using a for loop, and 'id(myobject) in (id(element) for
element in mylist)' is 2.4 times slower.

This is not a meaningful statement unless you at least qualify with the
number of item that are actually checked. For sufficently long sequences
both any() and the for loop take roughly the same amount of time over
here.

Sorry. I used short lists (a list of 20 floats) and the element
checked was not in the list.
(That was the case I usually deals with in my code.)

What is your (concrete) use case, by the way?

If you want efficiency you should use a dictionary instead of the list
anyway:

$ python -m timeit -s"d=dict((id(i) , i) for i in range(1000)); x =
1000" "id(x) in d"
1000000 loops, best of 3: 0.275 usec per loop

Peter

[bearophileHUGS]

Peter Otten:

PS: Take these numbers with a grain of salt, they vary a lot between runs.

Another possibility :-)
from itertools import imap
id(x) in imap(id, items)

>If you want efficiency you should use a dictionary instead of the list anyway:

I agree, but sometimes you have few items to look for, so building the
whole dict (that requires memory too) may be a waste of time.

In theory this may be faster to build, but in practice you need a
benchmark:
ids = set(imap(id, items))
followed by:
id(x) in ids

Bye,
bearophile